Irrelevant Baked texture maps
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You have three materials mapped to same UV space, so before you bake you need to tell Blender to bake each of those materials to the same image. (3 materials all baked to the same image).

To do that ......

1: In the UV editor Create a new image to bake to. In the screenshots below I named this image 3 material bake. save this Image to your hard drive.

2: Select one of the three materials.

In the Node Editor add a new Image Texture.
In this new node load up the image that was create in the UV editor. Leave this new node unconnected but selected! (Very important to leave it selected).

Select the second material and repeat (Add new Image Texture and load up same image to bake to as was done for the first material and leave it selected).

Repeat for third material.


3: In the 3D editor, Object mode select the model.
   Hit that Bake button.

All three materials should now be baked correctly    to the image that was created in step 1.